The present invention relates to reagent solutions of certain chromogenic crown ethers and to methods of using these reagent solutions for the extraction and spectrophotometric determination of potassium. The crown ether compounds which can be employed in the reagents have the structural formula: ##STR1## where R is taken from the group consisting of ##STR2## These crown ethers are systematically named 4'-(2",4"-dinitro-6"-trifluoromethyl-phenyl) aminobenzo-15-crown-5 and 4'-2",6"-dinitro-4"-trifluoromethylphenyl)aminobenzo-15-crown-5, respectively.
Selective reagents which permit the isolation of a particular ion or ions from a complex matrix are of economic interest. In particular, with respect to the potassium and sodium ions which are present in many biological systems, a reagent having the ability to selectively (and quantitatively) extract one of these ions in the presence of the other would be of interest in biochemical assays.
The ability of crown ethers to selectively extract alkali and alkaline earth metal ions has been recognized in the art [Pedersen, C. J., J. Am. Chem. Soc., 1967, 89, 7017]. Nonetheless, there has been little successful work in utilizing these compounds in analytical determinations until quite recently.
Of particular interest to the background of the present invention, is the description of the synthesis of a chromogenic crown like compound, 4'-picrylaminobenzo-15-crown-5, which was reported to be a selective extractant for potassium ions (10-800 p.p.m.) in the presence of sodium ions (2300 p.p.m.) [Takagi, M., Nakamura, H., and Ueno, K., Anal. Lett., 1977, 10, 1115]. Unlike the case with the reagents of the present invention, however, the 4'-picrylamino-benzo-15-crown-5 reagent suffers both from a poor extraction efficiency and from spectral overlap of its protonated (HL) and deprotonated (ML) species. That is to say, the reagent can be used to extract K.sup.+ from solution by forming a complex of the form. ##STR3## This (ML) compound, however, has a considerable spectral overlap with the unreacted reagent precursor (HL) compound.
Moreover, this picryl crown ether reagent does not readily quantitatively extract K.sup.+ in the presence of Na.sup.+. These same workers have reported efforts to improve the extraction efficiency of these reagent materials by adding bromo and nitro groups in the 5'position [Nakamura, H., et al, Talanta, 1978, 26, 921; Nakamura, H., et al, Anal. Chem., 1980, 52, 1668].